Method for sealing and fabricating cap for field emission display

ABSTRACT

Methods for sealing and fabricating a cap in an FED which is able to seal a cap in a vacuum space comprise: a step of fabricating a cap on which sealant is applied; a step of locating the cap with the sealant on a substrate of a panel on which a hole is formed in a vacuum chamber; a step of hardening the sealant by irradiating laser onto the sealant, in order to prevent oxygen from inducing into the panel, and to prevent electrodes formed on the panel from being contaminated.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a field emission display (FED),and particularly, to a method for sealing a cap and a method forfabricating the cap in a vacuumed space.

[0003] 2. Description of the Related Art

[0004] Recently, interests and importance of display is increased asmultimedia is developed. For example, a display having smaller weight,volume and power consumption is required in an environment where themobility is an important feature such as portable information devices,and a display having larger screen and angular field is required whenthe display is used as a media for transmitting information to themasses. Therefore, in order to satisfy the above requires, light andthin flat panel display should be developed. Cathode ray tube (CRT)which is mainly used as the display device presently has superiorfunction, however, volume and weight are increased as the screen isincreased, and has some problems such as high voltage and high powerconsumption.

[0005] Therefore, there are flat panel displays such as liquid crystaldisplay (LCD), plasma display panel (PDP), electro-luminescence (EL) andfield emission display (FED) for solving above problems.

[0006] Especially, the FED is a triode like the conventional CRT,however, the FED uses an acute cathode, not using a hot cathode. Thatis, a cold cathode, which emits electrons by quantum mechanical tunneleffect after concentrating high electric field on an emitter, is used.

[0007] Therefore, the electron emitted from the emitter is acceleratedby voltage applied between the anode and the cathode, and crashed onto aphosphor formed on the anode to radiate the phosphor. Therefore, the FEDhas a relatively simple electrode structure, and can be operated withhigh speed by using phosphor radiation due to electron beam, and hasadvantages such as full-color, full-grayscale, high brightness and highvideorate.

[0008]FIG. 1 is a perspective view showing a conventional FED.

[0009]FIG. 2 is a cross-sectional view showing the conventional FED.

[0010] As shown in FIGS. 1 and 2, the conventional FED comprises anupper glass substrate 2 and a lower glass substrate 8, a spacer 40 forsupporting vacuum space between the upper and lower glass substrates 2and 8, and a field emission array 32 formed on the lower glass substrate8.

[0011] The field emission array 32 comprises a cathode electrode 10 anda resistance layer 12 formed on the lower glass substrate 8, a gateinsulating layer 14 and an emitter 22 for emitting electrons formed onupper part of the resistance layer 12, and a gate electrode 16 formed onthe gate insulating layer 14.

[0012] The cathode electrode 10 supplies electric current to the emitter22, and the resistance layer 12 restricts overcurrent applied from thecathode electrode 10 toward the emitter 22 to supply even electriccurrent to the emitter 22.

[0013] The gate insulating layer 14 insulates between the cathodeelectrode 10 and the gate electrode 16. The gate electrode 16 is used asa fetch electrode for drawing electrons. The spacer 40 supports theupper and lower glass substrates 2 and 8 so as to maintain highlyvacuumed status between the upper and lower glass substrates 2 and 8.

[0014] In order to display an image, cathode voltage of negativepolarity (−) is applied to the cathode electrode 10, and anode voltageof positive polarity (+) is applied to the anode electrode 4. Therefore,when sufficient electric voltages are applied to the cathode electrode10 and the gate electrode 16, a strong electric field is generated, andelectrons 30 are emitted from a tip of the emitter 22 due to thegenerated electric field in quantum mechanical tunneling effect. Then,the emitted electrons 30 pass a hall of the gate electrode, and crashedonto phosphors of red, green and blue colors to excite the phosphors 6.At that time, visible ray of one of the red, green and blue colors isradiated according to the phosphor 6.

[0015]FIG. 3 is a cross-sectional view showing a conventional FED onwhich a focusing electrode is formed.

[0016] As shown therein, a focusing electrode 20 is formed on the gateelectrode 16 for focusing the electrons 30 emitted from the emitter 22.The focusing electrode focuses the electrons 30 by being appliedfocusing voltage of negative polarity (−). Also, a focusing insulatinglayer 18 is formed between the focusing electrode 20 and the gateelectrode 16.

[0017] As described above, the conventional FED requires highly vacuumedstatus in the panel greater than 10⁻⁶ Torr due to the operationalproperties. For example, a distance about sub-micron is maintainedbetween the gate electrode 16 and the emitter 22 and high electric fieldof 10⁷V/cm is applied therebetween. If the highly vacuumed status is notmaintained between the upper and lower glass substrates 2 and 8, theinsulation between the gate electrode 16 and the emitter 22 may bebroken. That is, neutral particles in the panel are crashed into theelectron beam and positive ions are generated. The positive ions aresputtered on the tip of the emitter 22 to degrade the device. Also, theelectrons 30 crashed with the neutral particles lose their energies, andtherefore, the electrons 30 can not sufficiently excite the phosphor 6,and thereby to lower the brightness.

[0018] Packaging processes of the conventional FED according to abovestructure will be described as follows.

[0019]FIG. 4 is a flow chart showing processing orders of vacuumpackaging the conventional FED using a vacuum pump in atmosphere.

[0020]FIG. 5 is an exemplary view showing process of installing a tubeand process of applying sealant for the conventional FED.

[0021] As shown in FIGS. 4 and 5, in the tube installing process, a fritglass is applied on the lower glass substrate 8 as a first sealant 52,and after that, a tube 50 is installed (ST2). At that time, the tube 50is installed on a hall 51 of the lower glass substrate 8.

[0022] After that, a spacer 40 is formed on the upper glass substrate 2,and the frit glass is dispensed and dried around the spacer as a secondsealant 54 (ST4). Herein, the second sealant 54 is installed to behigher than the spacer 40 as much as a predetermined distance (H1;usually 1 mm-2 mm), because the height of the frit glass is reducedabout 30-40% in preform sintering.

[0023] After the second sealant 54 is dispensed on the glass substrate2, the second sealant 54 is pre-sintered (ST6).

[0024]FIG. 6 is an exemplary view showing a process of preform sinteringthe sealant conventionally.

[0025] The preform sintering process has different sintering temperaturecurves according to frit materials in order to completely burn out abinder of organic material included in the frit glass. Generally, in theabove preform sintering process, a standard process is to hold thesecond sealant 54 for 30 minutes˜1 hour at about 300° C. temperature.After the second sealant 54 is preform sintered, the upper glasssubstrate 2 and the lower glass substrate 8 are compressed and alignedto adhere the substrates.

[0026] After that, the upper and lower glass substrates 2 and 8 aremoved to a heating chamber to sinter the first and second sealant 52 and54 (ST6).

[0027]FIG. 7 is an exemplary view showing a sealant sintering process inthe conventional art.

[0028] As shown therein, the sintering process is performed at thetemperature of 400° C.˜450° C. which is higher than that of the preformsintering after moving the panel into the heating chamber 70. At thattime, when the sintering process is performed under atmosphereenvironment, the cathode electrode 10, the gate insulating layer 14, thegate electrode 16, the emitter 22, the focusing insulation layer 18 andthe focusing electrode 20, which emit the electrons in the FED, may bedamaged by reacting with oxygen or carbon in the atmosphere. Especially,the metal material such as the emitter 22 can be oxidated easily, andtherefore, the luminous characteristic is lowered greatly.

[0029] In order to prevent the damage as above, inert gas 58 such asnitrogen and/or argon is supplied into the panel using a tube 56extended from the heating chamber 70, and therefore, devices of thefield emitting array are not reacted with the oxygen.

[0030] On the other hand, FIG. 8 is an exemplary view showing sealantsintering process according to the conventional art.

[0031] As shown therein, a gas inlet port 60 is formed on a lower endportion of the heating chamber 70 and a gas outlet port 62 is formed onan upper end portion of the heating chamber 70 to flow the inert gassuch as the nitrogen and/or the argon into the entire heating chamber70, and therefore, the materials for emitting the electrons can not bereacted with the oxygen in a high temperature process. Herein, theinducing of the inert gas is made by opening the inlet port for 10˜20minutes in the state that a valve out of the outlet port is closed tomake the inside of heating chamber 70 be the nitrogen and/or argon inertgas atmosphere, and after that, the outer valve is opened to flow thegas continuously.

[0032] Under above atmosphere, when the temperature of the panel ismaintained as 400˜450° C. temperature for 30 minutes˜1 hour, the firstand the second sealants are sintered and the panel sealing is completed.The above conventional sintering method is defined as a atmospheresealing method. At that time, the height of the second sealant 54 isextracted during the sintering process, and therefore, the height of thefrit glass is coincided with that of the spacer 40.

[0033]FIG. 9 is an exemplary view showing a getter inserted into theconventional tube.

[0034]FIG. 10 is an exemplary view showing a cutting process of theconventional tube.

[0035] As shown in FIGS. 9 and 10, after the upper glass substrate 9 andthe lower glass substrate 8 are attached, a getter 66 is inserted intothe panel through the tube 50 and the pumping process is performed(ST8). That is, as the panel, on which the upper glass substrate 2n andthe lower glass substrate 8 are attached, is heated in the heatingchamber 70, and at the same time, the inside of the panel is pumped by avacuum pump 72. Therefore, when the inside of the panel reaches to adesired vacuumed degree, middle portion of the tube 50 is heated by alocal heating device 68 to cut off the tube 50, and therefore, the panelis separated from the heating chamber 70.

[0036] On the other hand, in a pinch-off process for cutting off thetube 50, the tube 50 which is exposed in atmosphere is cut, andtherefore, the vacuumed degree of the panel is lowered. In order toincrease the vacuumed degree of the panel having lowered vacuumeddegree, high temperature is compressed to the getter 66 located in thepanel to activate the getter 66 (S12). When the getter 66 is activated,the vacuumed degree of the panel is increased more than a predeterminedlevel, and therefore, the final panel is completed.

[0037] However, the pinch-off process of the FED as described above isperformed under the atmosphere, the oxygen is induced through the hole51. Accordingly, when the oxygen is induced into the panel, the metalmaterial such as the emitter is easily oxidated, and thereby, the lifespan of the FED is reduced and the luminous characteristic is loweredgreatly. Also, color purities are different from respective points dueto the oxygen in displaying. Also, since the conventional panel sealingmethod is performed at high temperature, it takes a lot of times toprocess. Also, in the process of installing the tube, the tube isattached on the lower glass substrate 8 by the first sealant 52. At thattime, the electrodes formed on the lower glass substrate 8 may becontaminated by the organic binder of the first sealant 52.

SUMMARY OF THE INVENTION

[0038] Therefore, an object of the present invention is to provide amethod for sealing a cap in a field emission display (FED) which is ableto prevent oxygen from being induced into a panel by sealing a cap in avacuumed space.

[0039] Another object of the present invention is to provide a methodfor fabricating a cap of an FED which is able to prevent electrodesformed on a panel from being contaminated by removing an organic binderincluded in a sealant applied on a cap through a sintering process whenthe cap is fabricated.

[0040] To achieve the objects of the present invention, as embodied andbroadly described herein, there is provided a method for sealing a capin an FED comprising: a step of locating a cap, on which a sealant isapplied, on a substrate of a panel on which a hole is formed in a vacuumchamber; and a step of hardening the sealant by irradiating laser inorder to cover the hole.

[0041] Also, to achieve the objects of the present invention, there isprovided a method for fabricating a cap in an FED comprising: a step ofapplying a sealant on a substrate of glass material; a step of sinteringthe glass substrate on which the sealant is applied; and a step ofcutting the glass substrate on which the sealant is applied.

[0042] The foregoing and other objects, features, aspects and advantagesof the present invention will become more apparent from the followingdetailed description of the present invention when taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The accompanying drawings, which are included to provide afurther understanding of the invention and are incorporated in andconstitute a part of this specification, illustrate embodiments of theinvention and together with the description serve to explain theprinciples of the invention.

[0044] In the drawings:

[0045]FIG. 1 is a perspective view showing a conventional field emissiondisplay (FED) device;

[0046]FIG. 2 is a cross-sectional view showing the conventional FED;

[0047]FIG. 3 is a cross-sectional view showing a conventional FED onwhich a focusing electrode is formed;

[0048]FIG. 4 is a flow chart illustrating process orders of vacuumpackaging for the conventional FED using a vacuum pump in atmosphere;

[0049]FIG. 5 is an exemplary view showing a process of installing tubeand a process of applying sealant for the conventional FED;

[0050]FIG. 6 is an exemplary view showing a conventional sealant preformsintering process;

[0051]FIG. 7 is an exemplary view showing a conventional sealantsintering process;

[0052]FIG. 8 is an another exemplary view showing a conventional sealantsintering process;

[0053]FIG. 9 is an exemplary view showing a getter inserted into aconventional tube;

[0054]FIG. 10 is an exemplary view showing a processing of cutting tube;

[0055]FIGS. 11 through 13 are exemplary views showing a vacuum sealingmethod for an FED panel according to the present invention;

[0056]FIG. 14 is an exemplary view showing a method for sealing a cap inan FED according to an embodiment of the present invention;

[0057]FIGS. 15A and 15B are exemplary views showing laser radiated ontoa sealant of the cap;

[0058]FIG. 16 is an exemplary view showing hardening process of thesealant when the laser is radiated between the sealant of the cap and alower substrate according to the present invention; and

[0059]FIG. 17 is a view illustrating a method for fabricating the cap inthe FED according to the embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0060] Reference will now be made in detail to the preferred embodimentsof the present invention, examples of which are illustrated in theaccompanying drawings.

[0061]FIGS. 11 through 13 are exemplary views showing a vacuum sealingmethod for a field emission display (FED) panel according to the presentinvention.

[0062] As shown therein, a sealing application unit 110 comprises aframe 100, and a first and a second sealants 102 and 104 applied onupper/lower parts of the frame. Also, an upper glass substrate 106 and alower glass substrate 108 are aligned as taking the sealing applicationunit 110 therebetween. At that time, a getter 122 for maintaining ahighly vacuumed status by absorbing gas remained in a panel is insertedbetween the upper glass substrate 106 and the lower glass substrate 108.

[0063] The vacuum sealing method for the FED panel according to aboveconstruction will be described in detail as follows.

[0064] The frame 100 is made by a material having same thermal expansioncoefficient as those of the upper and lower glass substrates 106 and108. For example, the frame 100 can be made using glass material.

[0065] It is desirable that the first and second sealants 102 and 104are applied in a screen printing method rather than the conventionaldispensing method, because frit glass of lower viscosity flows down indispensing since the frit glass of paste type has the viscosity.

[0066] After that, the upper glass substrate 106, the lower glasssubstrate 108 and the sealing application unit 110 are moved to a vacuumchamber 120.

[0067] On the other hand, a plurality of auxiliary jigs, desirably, 4 ormore auxiliary jigs 112 are installed between the upper glass substrate106 and the lower glass substrate 108. The auxiliary jigs 112 maintain adistance between the upper glass substrate 106 and the lower glasssubstrate 108. After that, the upper and lower glass substrates 106 and108 are adhered by applying a predetermined load on the lower glasssubstrate 108. On the other hand, the above sealing method is defined asa vacuum sealing method.

[0068]FIG. 14 is an exemplary view showing a method for sealing a cap inthe FED device according to an embodiment of the present invention.

[0069] As shown in FIG. 14, the FED device of the present inventionundergoes cap sealing after the lower glass substrate 108 and the upperglass substrate 106 are adhered by the vacuum sealing method. That is,in the cap sealing method according to the present invention, a cap 136of glass material is disposed, and the cap 136, on which the sealant isapplied, is located on the substrate of the panel on which the hole isformed in the vacuum chamber. After that, the laser is radiated to thesealant 138 of the cap 136 to harden the sealant, in order to cover thehole.

[0070] On the other hand, as an another embodiment of the cap sealingmethod according to the present invention, the method of the presentinvention can be applied after attaching the lower glass substrate 8 andthe upper glass substrate 2 by the conventional atmosphere sealingmethod. At that time, there is no need to install the tube 50 as in theconventional atmosphere sealing method.

[0071] The cap sealing method for the FED according to the presentinvention will be described more specifically.

[0072] A plurality of spacers 144 for supporting the upper glasssubstrate 140 and the lower glass substrate 130 are installed in theupper and lower glass substrates 140 and 130 attached by the atmospheresealing method or by the vacuum sealing method. Also, the sealingapplication unit 110 is installed to attach the upper and lower glasssubstrate 140 and 130.

[0073] After that, the attached upper and lower glass substrates 140 and130 (hereinafter, referred to as “panel”) is moved to the vacuum chamber142. The vacuum chamber is exhausted to be vacuumed status of apredetermined Torr (desirably, 10⁻⁷ Torr) by a vacuum pump which is notshown, after the panel is moved therein. At that time, the inside of thepanel is also exhausted to be the vacuumed status.

[0074] After that, the cap 136 is aligned so as to cover the hole 132 ofthe lower glass substrate. That is, when the cap 136 is located on thehole 132 of the lower glass substrate by a robot arm (not shown) in thepanel of vacuum status, the hole 132 of the lower glass substrate andthe cap 136 are attached by the sealant of the cap.

[0075]FIGS. 15A and 15B are exemplary views showing radiation of thelaser onto the sealant of the cap.

[0076] As shown in therein, when the laser is radiated onto the sealant138 of the cap 136, the sealant 138 is hardened and the cap 136 isattached on the lower glass substrate 130.

[0077] For example, since the sealant 138 applied on the cap 136surrounds the cap 136 as a circular shape, the laser 146 is irradiatedon the sealant 138 as making a concentric circle. At that time, if adiameter of the sealant 138 applied on the cap 136 is small, most of thesealant is melted when the center part of the sealant is sintered. Onthe contrary, if the diameter of the sealant 138 applied on the cap 136,the laser 146 is irradiated onto the sealant 138 once more to attach thecap 136 on the lower glass substrate 130 completely. At that time, inorder to prevent the cap 136 or the upper and lower glass substrates 140and 130 from being damaged by local high temperature energy of the laser146, 200˜350° C. reference temperature is made. Accordingly, the damagesof the devices due to the oxidation or other thermal processes insintering for the cap sealing can be minimized, and also, the processingtime can be reduced as much as the difference of sintering temperature.

[0078]FIG. 16 is an exemplary view showing a process of hardening thesealant when the laser is radiated between the sealant and the lowersubstrate according to the present invention.

[0079] As shown therein, a first sealant portion 152 from the pointwhere the laser is radiated at first to the present point is a part thatis solidificated after melted by the energy, and a second sealantportion 154 on which the laser 146 is located is under process ofmelting. In addition, a third sealant portion 150 besides the aboveportions is under pre-sintered status which is not yet sintered by thelaser 146. Herein, the first sealant portion 152 is melted in a statethat the height is maintained as it is, and solidificated, andtherefore, the first sealant portion 152 maintains same height as thatof the third sealant portion 150 which is in preform-sintered status. Inaddition, even though the second sealant portion 154 is under theprocess of laser irradiation, the height of the second sealant portion154 is not affected because the first and third sealant portions supportthe second sealant portion 154.

[0080] However, if the laser irradiation speed is high, the firstsealant portion 152 on which the laser is irradiated may be distributedwidely,iated may be distributed widely,ction may be generated. At thattime, the laser 146 is irradiated as compressing the upper part of thecap 136 with a predetermined pressure by using a gap reference frame,and thereby, the height change of the cap 136 can be prevented.

[0081]FIG. 17 is a view showing a method for fabricating the cap in theFED device according to an embodiment of the present invention.

[0082] Referring to FIG. 17, a substrate 160 of glass material isdisposed. The substrate 160 of glass substrate is the same material asthose of the upper glass substrate 140 and the lower glass substrate130.

[0083] After that, a lot of sealant 138 is applied on the glasssubstrate 160 using a screen printing method as a circular shape. Atthat time, a center of circular sealant 138 is empty so that the sealant138 is not overlapped with the hole 132. Also, the sealant 138 of thecap is printed to be a few μm˜hundreds of μm thickness so as to absorbincident energy of the laser 146 easily and to maintain the vacuumedstate in the panel. Also, the frit glass is used as the sealant, and thefrit glass is formed by mixing glass powder and binder with more than10:1 mass ratio.

[0084] After the sealant 138 is printed on the glass substrate 160, theglass substrate 160 is sintered under 300˜400° C. At that time, theorganic binder component included in the sealant 138 is completely burntout and removed.

[0085] Finally, the glass substrate 160 is cut into predetermined sizesto complete the cap 136 on which the sealant is printed.

[0086] As described above, according to the method for sealing andfabricating the cap of the FED device of the present invention, the capis sealed in the vacuum space to prevent the oxygen from inducing intothe panel during the cap sealing process. Therefore, damages of thedevices due to the oxidation and other thermal processes can beminimized, and color purity can be maintained constantly on respectivepoints of the panel in displaying. Also, the processing time can bereduced less than that of the conventional sintering process as much asthe difference of sintering temperatures. Also, there is no need toinstall the tube on the panel as in the conventional sintering method,and therefore, the process becomes simple and the processing time can bereduced. In addition, the organic binder included in the sealant appliedon the cap is removed through the sintering process when the cap isfabricated, and therefore, the contamination of the electrodes formed onthe lower glass substrate can be prevented.

[0087] As the present invention may be embodied in several forms withoutdeparting from the spirit or essential characteristics thereof, itshould also be understood that the above-described embodiments are notlimited by any of the details of the foregoing description, unlessotherwise specified, but rather should be construed broadly within itsspirit and scope as defined in the appended claims, and therefore allchanges and modifications that fall within the metes and bounds of theclaims, or equivalence of such metes and bounds are therefore intendedto be embraced by the appended claims.

What is claimed is:
 1. A method for sealing a cap in a field emission display (FED) comprising: a step of locating a cap, on which a sealant is applied, on a substrate of a panel on which a hole is formed in a vacuum chamber; and a step of hardening the sealant of the cap by irradiating laser in order to cover the hole.
 2. The method of claim 1, wherein the panel is exhausted to be a predetermined Torr by a vacuum pump in the vacuum chamber.
 3. The method of claim 2, wherein the vacuumed state is formed to be 10³¹ ⁷ Torr.
 4. The method of claim 1, wherein frit glass is used as the sealant, and the frit glass is formed by mixing glass powder and binder with more than 10:1 mass ratio.
 5. The method of claim 1 further comprising: a step of forming a sealing application unit by applying the sealant on a frame; a step of aligning an upper substrate and a lower substrate as taking the sealing application unit therebetween; and a step of moving the upper substrate, the lower substrate and the sealing application unit into the vacuum chamber.
 6. The method of claim 5, wherein the frame uses a material having same coefficient of thermal expansion as those of the upper substrate and the lower substrate.
 7. The method of claim 5, wherein the sealing application unit is made by applying respective sealant on upper and lower parts of the frame in a screen printing method.
 8. The method of claim 1, wherein the cap has a bottom surface on which the sealant is applied and is formed to cover the hole of the panel.
 9. The method of claim 8, wherein the cap is located to cover the hole on the lower glass substrate of the panel by a robot arm.
 10. The method of claim 1, wherein the cap is same material as those of the upper and lower substrates.
 11. The method of claim 1, wherein the cap is a glass material having a coefficient of thermal expansion which is little different from those of the upper and lower substrates.
 12. The method of claim 1, wherein reference temperature is maintained to be 200˜350° C. in the sealant hardening process.
 13. The method of claim 1, wherein the cap is compressed by a predetermined pressure so that a height of the cap can be maintained constantly when laser is irradiated onto the sealant of the cap.
 14. The method of claim 1, wherein a predetermined pressure is compressed on the panel using a gap reference frame in case that the height of the sealant is changed since the sealant of the cap is melted when the laser is irradiated.
 15. The method of claim 1, wherein the panel comprises: a plurality of spacers supporting the upper glass substrate and the lower glass substrate; and a sealant for attaching the spacers and the upper and lower glass substrates.
 16. The method of claim 1, wherein the panel includes auxiliary jig for maintaining a distance between the upper glass substrate and the lower glass substrate constantly.
 17. A method for fabricating a cap in an FED comprising: a step of applying sealant on a substrate of glass material; a step of sintering the substrate of glass material on which the sealant is applied; and a step of cutting the substrate of the glass material on which the sealant is applied.
 18. The method of claim 17, wherein the substrate of glass material on which the sealant is applied is sintered under 300˜400° C. temperature to remove organic binder component included in the sealant.
 19. The method of claim 17, wherein the sealant is applied to be a few μm˜hundreds of μm thickness.
 20. The method of claim 17, wherein the sealant is applied to be a circular shape on the substrate of glass material using a screen printing method, and is not applied on a center of the substrate of glass material. 